Urolithiasis, Calcium Phosphate

Basics

Overview

Calcium phosphate (CP) uroliths within the urinary tract and associated clinical conditions.
Forms of CP identified in dogs and cats include: calcium phosphate apatite (CAP), calcium phosphate carbonate (CPC), calcium hydrogen phosphate dihydrate (Brushite), and uncommon minerals-tricalcium phosphate (whitlockite) and octacalcium phosphate.
CP uroliths represent a small fraction (dogs <1.0%, cats <0.5%) of all uroliths submitted to the MUC from 1981 to 2013 (Table 1).

A greater percentage of CP uroliths are found in the upper tract (kidney and ureter) (2.5% cats, 3.5% dogs), than in the lower tract (bladder and urethra).
CP uroliths (excluding brushite) do not have a characteristic shape. Brushite uroliths are typically round and smooth.
Color of CP uroliths are usually cream or tan. Blood clots mineralized with CP are typically black.

Signalment

Signs

Depend on location, size, and number of uroliths.
Pollakiuria, dysuria, hematuria, and urethral obstruction.
Nephroureteroliths-typically asymptomatic but may have persistent hematuria or signs referable to concomitant renal failure (primarily cats).

Causes & Risk Factors

CPC-commonly a minor component of struvite and calcium oxalate uroliths.
Pure CP uroliths-usually associated with metabolic disorders, e.g., primary hyperparathyroidism, renal tubular acidosis, and excessive dietary calcium and phosphorus.
Urinary tract infections-Increased calcium excretion in combination with urinary tract infection with urease producing bacteria may be risk factors favoring CPC.
Nephroliths, urocystoliths, and urethroliths composed of blood clots mineralized with CP suggest dystrophic mineralization of tissue, in contrast to metastatic mineralization reflecting abnormal calcium and phosphorus metabolism.
Other risk factors-see Urolithiasis Calcium Oxalate.

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

Other common causes of hematuria, dysuria, and pollakiuria, with or without urethral obstruction, include urinary tract infection and urinary tract neoplasia.
Other radiodense uroliths-magnesium ammonium phosphate, calcium oxalate, and silica.
Meta-static or dystrophic mineralization of urinary tract parenchyma may resemble uroliths.

CBC/Biochemistry/Urinalysis

Usually unremarkable.
Hypercalcemia or azotemia rarely detected; post-renal azotemia in some animals with complete urinary outflow obstruction.
Urinary sediment analysis-amorphous crystals in some patients; brushite (calcium hydrogen phosphate dihydrate) forms are elongated, rectangular, lath-shaped crystals.

Other Laboratory Tests

Quantitative analysis of retrieved uroliths is necessary to confirm their mineral composition.
Serum concentrations of parathyroid hormone, parathyroid hormone–related peptide, and hydroxycholecalciferol may help establish underlying causes.

Imaging

CP uroliths are radiodense (as dense as bone) and are often detected by survey radiography.
CP uroliths may be detected by ultrasonography.

Other Diagnostic Procedures

CP uroliths in the urethra and bladder may be detected by cystoscopy.

Treatment ⬆ ⬇

Treatment

Medical dissolution of CP uroliths remains a goal for the future.
Consider surgical removal of lower tract uroliths that cannot be removed by minimally invasive procedures (e.g., voiding urohydropropulsion, basket retrieval, intracorporeal lithotripsy, laparoscopic cystotomy).
Avoid performing disfiguring urethrostomies by using basket retrieval to remove urethroliths, retrograde urohydropropulsion to flush urethroliths into the bladder or using lithotripsy to fragment urethroliths.
Shock wave lithotripsy is an alternative to surgery for removal of nephroliths, ureteroliths, and urocystoliths in dogs.
Correction of hyperparathyroidism or other causes of hypercalcemia should minimize further urolith formation.

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

Radiography after surgery to verify complete urolith removal is a standard of practice.
Abdominal radiography or ultrasonography every 3–5 months to enhance early detection of urolith recurrence and prevention of the need for repeated surgery.
Small uroliths are easily removed by voiding urohydropropulsion or catheter retrieval.

Prevention/Avoidance

A high-moisture (canned) diet formulated to prevent formation of calcium oxalate uroliths may help prevent recurrence.
Foods for older dogs that are lower in protein, phosphorus, and calcium, and which do not promote acidic urine are usually associated with decreased calcium and phosphorus excretion. Hill's Prescription canned g/d is one example.
Because of the high moisture content of canned foods and their tendency to promote dilute urine, canned diets are more effective than dry diets in preventing recurrence.
Avoid excessive acidification or alkalinization of urine.